UGA is recoded as selenocysteine due to presence of
Recoding UGA from stop to selenocysteine relies on cis-acting SECIS element forming kink-turn hairpin structure with conserved quartet UGA, AUGA, and apical AAR motifs. Eukaryotic SECIS resides in 3' UTR; bacterial SECIS immediately downstream within coding region positioned to contact SelB. SECIS recruits trans-acting factors SBP2 and EFSec or SelB, positioning Sec-tRNASec near ribosomal A-site and antagonizing release factor access. Interaction involves ribosomal protein L30 and SBP2 Lys-rich domain, prolonging pause at UGA and favoring incorporation. Without SECIS, UGA triggers standard termination via eRF1. Hence RNA structure contextually modifies codon semantics enabling regulated selenoprotein expression.
Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 6: SECIS Element Mediated UGA Recoding